Swift filter array of strings

I've had troubles filtering array of keywords (strings) in swift ,My code:

   self.filteredKeywords=filter(keywords.allValues, {(keyword:NSString) ->                                              
     Bool in
        let words=keyword as? NSString
        return words?.containsString(searchText)
    })

As AnyObject can't be subtype of NSString ,i am stuck in this code !